This invention relates to memory devices in general and in particular to random access memory devices. Still more particularly, this invention relates to random access memory devices which utilize phase change materials as the storage medium. Random access memories are well known in the prior art. Early semiconductor experiments in the storage and random retrieval of digital data have led to an entire industry which specializes in the manufacture of such memory devices. Typically, a semiconductor memory device operates by storing and retrieving an electrical charge from a plurality of identifiable locations.
Various other techniques are also utilized in the construction of random access memories. As an example, it is quite common to utilize magnetic domains to store and retrieve a sequence of digital data. Early and bulky core memories have given way to magnetic disks which utilize much smaller magnetic domains to store millions of bits of digital data on small "floppy" disks. While the storage capacity of these magnetic disks is greatly enhanced over known semiconductor memories, the delicacy of these disks and their relatively slow retrieval rates have relegated this technique to applictions where speed is not a factor.
More recently, optical storage media have been utilized for high data rate applications. Initially, such optical data storage disks were utilized as read only memories and were primarily utilized to store the multiple frames of data necessary to reconstruct a video picture. The increases in laser technology which have occurred in conjunction with these disks and the rapidity with which an optical system can function have made the optical storage device a likely candidate for high data rate random access memories. Many attempts have been made in recent years to create an optical erasable access memory utilizing materials which will respond in a different manner to different wavelength lasers or which can be switched, by an external stimulus, from one optical state to another.
An excellent example of this type of technology may be seen in U.S. Pat. No. 4,236,156, issued to the inventor herein. This patent discloses a technique for writing and erasing data into a thin film of thermochromic or pressure sensitive film utilizing a combination of lasers and/or surface acoustic waves to affect an optically discernible phase change in the film material.
Another example of this type of technology is disclosed in U.S. Pat. No. 4,009,488, issued to Ronald S. Smith. The Smith patent discloses a video technology which includes a stress wave propagating apparatus which defines a viewing plane and includes a device for initiating propagation of stress waves in a first scanning direction. A light emitting apparatus is utilized to provide a second scan line and the emitted light is modulated by a video signal and interacts with the stress wave to result in a video display. These early designs have been proven to be complex and bulky systems which often rely on extensive support circuitry to operate.